Determination of low boiling constituents of a liquefied normally gaseous mixture



Jan. 18, 1966 SMITH 3,229,504

DETERMINATION OF LOW BOILING CONSTITUENTS OF A LIQUEFIED NORMALLY GASEOUS MIXTURE Filed Aug. 27, 1962 VAPOR PRESSURE OF PROPANE-ETHANE AND PROPANE-PROPYLENE MIXTURES AT |OOF 9 240- Q INITIAL 1 ETHAN E PROPANE PROPANE PROPYLENE w INITIAL (LG L/ E 2I0- FINAL 0 200- FINAI ETHANE+PROPANE Q [90 I I I I I I I I I I ETHANE o I a 3 4 5 6 7 s 9 o 5 I0 I5 3o 4o 5o PROPYLENE CONCENTRATION INVENTOR.

D. E. SMITH A TTORNE VS United States Patent 3,229,594 DETERMINATIGN 0F LQW BOILING CONSTITU- ENTS 6F A LIQUEFIED NORMALLY GASEOUS MDKTURE Dan E. Smith, Bartlesville, Olden, assignor to Phillips Petroleum Company, a corporation of Delaware Filed Aug. 27, 1962, Ser. No. 219,541 3 Claims. (Cl. 73-53) This invention relates to the determination of the concentration of low boiling constituents of a liquefied normally gaseous mixture. In one aspect, the invention relates to the method and apparatus for the determination of the concentration of at least one normally gaseous component of a multi-component, liquefied, normally gaseous mixture.

Commercial liquefied petroleum gases (LPG), comprising principally propane and butane, are Well known and have wide utility as fuel. Since these materials contain normally gaseous components, it is necessary to compress the mixture so as to form and maintain a liquid. In order to minimize the compressor loading in transferring the LPG to vehicles for transportation and the like, it is desirable to maintain the ethane and lighter boiling components as a minimum concentration. If the ethane and lighter concentration increases, the vapor pressure of the mixture increases and thus greater horsepower is required for compression. Since these liquefied gases are transported and handled in a variety of containers and by non-technical personnel, it is desirable to have a method and apparatus for the determination of the ethane and lighter concentration which is simple to operate and requires a minimum of calculations.

It is an object of the invention to provide a method and apparatus for the determination of the concentration of at least one normally gaseous component of a liquefied mixture of normally gaseous materials.

It is another object of the invention to provide method and apparatus for the determination of the concentration of ethane and lower boiling components of a multi-component, liquefied, normally gaseous mixture comprising a tEtjOl proportion of propane and a minor proportion of butane, ethane and lower boiling components.

These and other objects of the invention will be readily apparent to those skilled in the art from the accompanying disclosure, the drawing and the claims.

This invention is best accomplished employing apparatus comprising an enclosed vessel, an inlet conduit for the introduction of a liquid mixture into said vessel, said mixture comprising a mixture of normally gaseous materials under pressure, means for Withdrawing a predetermined amount of said mixture from said vessel thereby creating a vapor space, pressure sensing means for determining the vapor pressure of said liquid mixture, an outlet conduit, and means for withdrawing a predetermined amount of vapors from said vapor phase.

In one embodiment, the concentration of normally gaseous components boiling below a predetermined temperature in a mixture of liquefied normally gaseous material is determined by introducing said mixture into a confined zone until said zone is liquid full, removing a portion of said liquid mixture to provide a vapor phase in equilibrium with the resultant liquid phase, releasing a portion of said vapors from said vapor phase, terminating said release and after again establishing equilibrium determining the vapor pressure in the vapor phase, said vapor pressure being proportional to the concentration of said first normally gaseous components.

This invention is best described by reference to the accompanying drawing.

FIGURE 1 is a schematic illustration of apparatus suitable for the practice of the invention.

3,229,564 Patented Jan. 18, 1966 FIGURE 2 is a plot of the vapor pressures of propaneethane and propane-propylene mixtures at F. and is hereinafter described with reference to the example.

FIGURE 1 represents an enclosed vessel of a suitable material which is impermeable to the transmission of liquid or vapors therethrough and Whose interior surfaces are impervious to attack by the liquefied mixture. For purposes of simplification, the apparatus will be described with reference to the determinatioin of the concentration of ethane and lighter boiling components in an LPG mixture comprising ethane and lighter components, butane and propane, with the majority of the components being propane. A typical LPG mixture comprises 90 weight percent propane, 4-8 Weight percent ethane and 2-6 weight percent butane. So that all measurements may be equated, it is necessary that the temperature at the time of all determinatioins be known, and that the vapor and liquid phases be in equilibrium at the time of determination of pressures and temperatures. A suitable method for accomplishing this objective is to employ a water bath or the like except during periods of withdrawal or injection of fluid.

A mixture of ethane and propane of known concentrations is introduced into vessel 1 through conduit 5, valve 7 and conduit 9 until the interior of said vessel is completely full. Residual vapor present in vessel 1 during the introduction of material thereto is exhausted by venting through conduit 27 and valve 29. Valves 13 and 21 are closed during this injection. Valve 7 is then closed and valve 13 is opened with the vessel in a vertical position so that the liquid in the upper portion of the interior of the vessel may drain therefrom through eduction tube 11 or other suitable means and out through valve 13 and conduit 15. With the container in the vertical position, the valve remains open until no liquid continues to be removed therefrom and valve 13 is then closed, thus causing an interface 17 with the liquid phase and a vapor phase at the upper terminus of the eduction tube. The entire vessel is then permitted to remain inactive until the liquid and vapor phases have had an opportunity to come into equilibrium at the prevailing conditions of temperature. Any temperature above the boiling point of the low boiling constituents is desirable. To maintain a desired uniform temperature for establishing equilibrium vessel 1 is immersed in a temperature controlled Water bath 25. It has been found for a mixture comprising ethane and propane that a temperature in the range of 0 F. to F. is satisfactory. When the vapor-liquid phases have reached equilibrium, as indicated by the pressure gauge 3 in the upper portion of the vapor phase, valve 21 is rapidly opened, thus permitting the rapid escape of vapors through conduit 19, valve 21 and orifice 23 for a known interval of time so as to remove a known quantity of vapor. In this specific instance, the gas was vented through an ordinary brass A Hoke 309 valve with a V-stem point having an orifice of 0.059 inch. After a predetermined period of time which is inversely proportional to the orifice area, e.g., 10 seconds, the valve is closed and the vapor and liquid phases are again permitted to attain equilibrium at the same temperature as initially. The pressure is then determined. The pressure prior to Withdrawal of the vapor phase through conduit 19 is known herein as the initial pressure and the vapor pressure subsequent to the withdrawal is known herein as the final pressure. This final vapor pressure is indicative of the concentration of the ethane and lower boiling constituents in the initial mixture. In this specific embodiment, a series of determinations were made for both a mixture of ethane and propane and a mixture of propane and propylene at known concentrations as tabulated in Table I.

Table I These determinations were then plotted as FIGURE 2. It will be seen that there is a direct proportional relationship between the final vapor pressure and the ethane or propylene concentration. With this formulation and tabulation, it is then possible to employ the apparatus as a means and method for the determination of the concentration of an unknown mixture. In such a procedure, the unknown mixture is introduced into vessel 1, treated as hereinbefore described and the final vapor pressure determined. The concentration of ethane and lighter boiling constituents in the original mixture is then readily determinable from FIGURE 2. For instance, if the final vapor pressure of the unknown mixture should be determined as 200 p.s.i.g., it will be seen from FIGURE 2 that the concentration of ethane and lighter boiling components is approximately 7.1 weight percent. Similar graphs or tabulations may be made for diflerent admixtures. It is only necessary that the control be based on an admixture similar to that of the unknown. For example, it would be important to know that ethane and lighter boiling components are present, but it would not be necessary to know what the lighter boiling components are. Similarly, it would not be necessary to know whether heavier boiling components than propane are present.

It has been found that it is important that the amount of vapor removed from the vapor space is constant for the control and the run. To negate the importance of variables such as solubility of the components at different temperatures, it is preferable to employ an orifice of the same diameter for both the control run and the experimental run. In other words, it is advisable to calibrate each instrument separately and independently for each type of mixture to be analyzed. It is preferred that the orifice through which the vapor is permitted to escape be in the range of 0.01 to 0.1 inch in diameter and that the time interval for escape of the vapors, although de pendent on the volume within the container and the orifice area, be within the range of 5 seconds to 60 seconds. Any temperature may be employed which is above the boiling point of the minor component.

While certain examples, structures, composition and process steps have been described for purposes of illustration, the invention is not limited to these. Variation and modification within the scope of the disclosure and the claims can readily be effected by those skilled in the art.

I claim:

1. A process for the determination of the concentration of a normally gaseous component boiling below a predetermined temperature in a mixture of liquefied normally gaseous material comprising introducing said mixture into a confined zone until said zone is liquid full, removing a portion of said liquid mixture to provide a vapor phase in said zone, adjusting the temperature of said confined zone and its contents to said predetermined temperature, establishing equilibrium of said vapor phase with the liquid phase determining a first vapor pressure, releasing a portion of said vapors from said vapor phase, terminating said release and after again adjusting the temperature of said confined zone and its contents to said predetermined temperature and establishing equilibrium determining a second vapor pressure, said second vapor pressure being related to the concentration of said normally gaseous component.

2. A process for the determination of the concentration of ethane and lower boiling components of a multicomponent, liquefied, normally gaseous mixture comprising a major proportion of propane and a minor proportion of butane, ethane and lower boiling components, comprising introducing said mixture into a confined zone until said zone is liquid full, removing a portion of said liquid from said zone to provide a vapor phase in said zone, adjusting the temperature of said confined zone and its contents to a desired value, establishing equilibrium of said vapor phase with the resultant liquid phase, determining a first vapor pressure, thereafter releasing a portion of said vapors from said vapor phase, terminating said release, adjusting the temperature of said confined zone and contents to said desired value, establishing equilibrium between said vapor and liquid phases, and determining the final vapor pressure of said vapor phase, said vapor pressure being related to the concentration of said ethane and lower boiling components in said mixture.

3. The process of claim 2 wherein said vapors are released for a period of time in the range of 5 seconds to seconds.

References Cited by the Examiner UNITED STATES PATENTS 2,594,683 4/1952 Rolfson 7317 2,782,628 2/1957 Jacobs et al. 7353 2,847,852 8/1958 Rhodes et al. 73-53 3,037,375 6/1962 Jacobs et al. 73-53 LOUIS R. PRINCE, Primary Examiner.

JOSEPH P. STRIZAK, RICHARD QUEISSER,

Examiners. 

1. A PROCESS FOR THE DETERMINATION OF THE CONCENTRATION OF A NORMALLY GASEOUS COMPONENT BOILING BELOW A PREDETERMINED TEMPERATURE IN A MIXTURE OF LIQUEFIED NORMALLY GASEOUS MATERIAL COMPRISING INTRODUCING SAID MIXTURE INTO A CONFINED ZONE UNTIL SAID ZONE IS LIQUID FULL, REMOVING A PORTION OF SAID LIQUID MIXTURE TO PROVIDE A VAPOR PHASE IN SAID ZONE, ADJUSTING THE TEMPERATURE OF SAID CONFINED ZONE AND ITS CONTENTS TO SAID PREDETERMINED TEMPERATURE, ESTABLISHING EQUILIBRIUM OF SAID VAPOR PHASE, WITH THE LIQUID PHASE DETERMINING A FIRST VAPOR PRESSURE, RELEASING A PORTION OF SAID VAPORS FROM SAID VAPOR PHASE TERMINATING SAID RELEASE AND AFTER AGAIN ADJUSTING THE TEMPERATURE OF SAID CONFINED ZONE AND ITS CONTENTS TO SAID PREDETERMINED TEMPERATURE AND ESTABLISHING EQUILIBRIUM DETERMINING A SECOND VAPOR PRESSURE, SAID SECOND VAPOR PRESSURE BEING RELATED TO THE CONCENTRATION OF SAID NORMALLY GASEOUS COMPONENT. 